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  1. #21
    Join Date
    Mar 2006
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    Quote Originally Posted by Ariadne View Post
    What he said!

    Outside of Europe (including Greece) the charts on your plotter won't always match up to whats there. So I suggest you all practice using Eyeball Mk1 and confriming what actually see then cross check with the plotter.

    Up the Gambia river the chart (609) is 1942 vintage so it doesn't work at all with GPS, but ...
    Plotter right 99% of the time but you can have problems closer to home than the Gambia...
    Entering the Morlaix estuary I would have hit the Chateau de Taureau rocks if I had believed my plotter (Raymarine/Navionics) and further up the river (and the Trieux river) I found GPS/Plotter unreliable, showing you up the hill.

    Moral is to keep your wits about you and avoid tight entrances in fog unless you can rely on other methods e.g. radar, depth-sounder etc.

  2. #22
    Join Date
    May 2001
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    29,103

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    Quote Originally Posted by multihullsailor6 View Post
    I now always put a waypoint in the harbour entrance and if in doubt follow the instruments.
    Just check the chart is WGS84 or you might pile into the harbour wall! I once set a waypoint between a pair of lateral buoys inside the Goodwin Sands and was surprised to see the autopilot take me well outside the gate
    One hull good, two hulls better.

  3. #23
    Join Date
    Feb 2003
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    8,447

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    Quote Originally Posted by Nostrodamus View Post
    Well I am a bit fed up with red diesel posts at the moment so how about something else.
    I am sure this has happened to most people at sometime or another.
    You make a passage plan, check the almanac and cruising guide or maybe you are going into somewhere familiar or unfamiliar but at night. You check your chart plotter and the charts which seem to be working fine but what you see does not seem to match up. You check again and also check depth on the sounder against the chart which also match but end up confused thinking you are heading for the wrong light or even the wrong harbour.
    How much do you trust your plotter and GPS which tell you one thing against a tired brain that is confused.
    Do you follow the chart plotter a little further towards that light with images of moths going through your head. There is little else around to get a bearing from.
    I was in Pwllheli for 10 years, thought i knew the entrance like the back of my hand. Twice doing a night passage back I have thought I was going crazy and twice did a 180 at the entrance! Brain wouldnt believe the plotter, it was right!
    Stu

  4. #24
    VicMallows is offline Registered User
    Location : Emsworth, Chichester Hbr, UK
    Join Date
    Nov 2003
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    3,233

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    Quote Originally Posted by hrchivers View Post
    One occasion last year I sailed through the main passage on the Portsmouth submarine barrier while the chart plotter definately had me over the barrier to the North. Others have since told me of the same experience here, although it may just be that the narrow gap makes us aware of errors within the normal possible range for GPS.
    But if you 'mark' it when going through in clear daylight you should be pretty safe in future when conditions are poor.

    Vic

  5. #25

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    Quote Originally Posted by skipper_stu View Post
    I was in Pwllheli for 10 years, thought i knew the entrance like the back of my hand. Twice doing a night passage back I have thought I was going crazy and twice did a 180 at the entrance! Brain wouldnt believe the plotter, it was right!
    Stu
    I am with you on this one Stu, especially when you are tired. You may of gone into the same place many times before but the brain can do strange things. There may be no reason to doubt the chart plotter but for some reason your brain tells you things are different.

  6. #26
    Join Date
    Nov 2009
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    13,151

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    Quote Originally Posted by Nostrodamus View Post
    How much do you trust your plotter and GPS which tell you one thing against a tired brain that is confused.
    Would always rely on the gps and plotter rather than myself. I can get tired and maybe sea sick and I'm careless. Plotter isnt.

    But the odd thing is I much prefer navigating using paper. So I check my paper against the plotter and if there is a difference I re do my paper work.

  7. #27
    Join Date
    Jul 2003
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    1,550

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    Harbours and entrances, rivers etc is where most people will have trouble with plotters. Admiralty charts are generally accurate at sea and along coastline but extremely poor in close. Locally the charts are only slightly adjusted from Captain Cooks originals. The small print states the error to be used when navigating electronically for my local chart which includes Sydney harbour as 0.11 minutes Southward and .08 minutes Eastward. If your using Seaclear then its a matter of allowing the corrections when calibrating. With bought plotters using Admiralty charts under license do they make these adjustments?
    Radar will only display line of sight. If the coastline or harbour has cliffs or other obstructions, much of the detail will be hidden. At a distance it is not the coastline your looking at sometimes but halfway up a hill etc. Radar is handy but no match for a mark one eyeball and common sense.
    For my home port I have a chart supplied from the Waterways [non Admiralty] which is extremely accurate, its what they use for plotting all the moorings and placing navigation markers. Seaclear on a laptop and a 48 channel gps puck I can safely go anywhere on that chart and be exactly in the right place. Other charts and particularly Admiralty charts in places like Sydney harbour I would not be confident.

  8. #28
    Join Date
    Jan 2004
    Posts
    5,169

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    With the huge propensity for the brain to see what it expects to see I have a fair degree of confidence in the laptop. I mean chart plotter. If the charts/gps talley up with something like radar or a shoreside AIS station then even better. And if those agree with the gps location on google earth then hurrah! Gps on the laptop chartplotter have had me sailing up a main road and anchoring in a hotel (nice one, mind you) but google earth has yet to get it badly out.

    But the brain? Well, clever though it is, it can be a bit too sure of itself.

  9. #29
    Join Date
    Nov 2009
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    13,151

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    Quote Originally Posted by fishermantwo View Post
    Harbours and entrances, rivers etc is where most people will have trouble with plotters. Admiralty charts are generally accurate at sea and along coastline but extremely poor in close. Locally the charts are only slightly adjusted from Captain Cooks originals. The small print states the error to be used when navigating electronically for my local chart which includes Sydney harbour as 0.11 minutes Southward and .08 minutes Eastward. If your using Seaclear then its a matter of allowing the corrections when calibrating. With bought plotters using Admiralty charts under license do they make these adjustments?
    Radar will only display line of sight. If the coastline or harbour has cliffs or other obstructions, much of the detail will be hidden. At a distance it is not the coastline your looking at sometimes but halfway up a hill etc. Radar is handy but no match for a mark one eyeball and common sense.
    For my home port I have a chart supplied from the Waterways [non Admiralty] which is extremely accurate, its what they use for plotting all the moorings and placing navigation markers. Seaclear on a laptop and a 48 channel gps puck I can safely go anywhere on that chart and be exactly in the right place. Other charts and particularly Admiralty charts in places like Sydney harbour I would not be confident.
    Over here we have been going through a process of re-basing Admiralty charts onto the WGS datum from the old OSGB one. At the same time, satellite mapping has allowed corrections to be made for the inaccuracies in the old surveys. For example, rocks in harbour entrances were surveyed position wise from land and if that land wasnt where we thought it was using sextants etc, then neither were the rocks on the charts. There were some bits of the UK that were as much as half a mile away from where they were supposed to be, but now all is OK.

    I suspect that now you are grown up and your own country the admiralty will be relying on you to do the same sort of exercise. Even if we had a Navy of any consequence, we could hardly send a navy surveying boat into Oz waters . You might get uptight.

    Having said all that, the skipper who uses a GPS to navigate through the harbour mouth is an idiot anyway.

  10. #30
    Join Date
    Jul 2003
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    1,550

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    Quote Originally Posted by Bosun Higgs View Post
    Over here we have been going through a process of re-basing Admiralty charts onto the WGS datum from the old OSGB one. At the same time, satellite mapping has allowed corrections to be made for the inaccuracies in the old surveys. For example, rocks in harbour entrances were surveyed position wise from land and if that land wasnt where we thought it was using sextants etc, then neither were the rocks on the charts. There were some bits of the UK that were as much as half a mile away from where they were supposed to be, but now all is OK.

    I suspect that now you are grown up and your own country the admiralty will be relying on you to do the same sort of exercise. Even if we had a Navy of any consequence, we could hardly send a navy surveying boat into Oz waters . You might get uptight.

    Having said all that, the skipper who uses a GPS to navigate through the harbour mouth is an idiot anyway.
    They are called Admiralty charts but not your Admiralty! The navy section responsible is just a few miles away at Wollongong.

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